Compositions which undergo decomposition upon exposure to light to produce a free radical (free radical initiators) have been well-known in the field of light-sensitive recording materials. These compounds have been widely used as photopolymerization initiators which are contained in photopolymerizable compositions, light activators which are contained in free radical-containing photographic compositions, or initiators of acid catalyzation occurring upon exposure to light. Such free radical initiators are used to prepare various light-sensitive materials useful in printing materials, reproducing materials, copying materials and other recording materials.
As a halogen free radical initiator sensitive to wavelengths in the ultraviolet to visible light range, there has been proposed a halomethyl-S-triazine compound. Such a compound is further described in U.S. Pat. Nos. 3,954,475, 3,987,037, and 4,189,323, and JP-A-62-58241 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Although such a compound is sensitive to wavelengths in the ultraviolet to visible light range, it has some difficulty remaining stable over time. Furthermore, such a compound becomes light yellow colored upon exposure. Therefore, it can hardly be used in color proofs which normally require strict color hue reproducibility.